This invention relates to a semiconductor memory devices and more particularly to an improved sense amplifier for an electrically-erasable, electrically-programmable, read-only-memory (EEPROM) memory.
An EEPROM memory cell typically comprises a floating-gate field-effect transistor. The floating-gate conductor of a programmed memory cell is charged with electrons, and the electrons in turn render the source-drain path under the charged floating gate nonconductive when a predetermined voltage is applied to the control gate. The nonconductive state is read by a sense amplifier as a "zero" bit. The floating-gate conductor of a non-programmed cell is neutrally charged (or slightly positively or negatively charged) such that the source-drain path under the non-programmed floating gate is conductive when the predetermined voltage is applied to the control gate. The conductive state is read by a sense amplifier as a "one" bit.
Each column and row of an EEPROM array may contain thousands of floating-gate memory cells. The sources of each cell in a column are connected to a source-column line and the source-column line for a selected cell may be connected to reference potential or ground during reading of the selected cell by a sense amplifier. The drains of each cell in a column are connected to a separate bitline (drain-column line) and the drain-column line for a selected cell is connected to the input of the sense amplifier during reading of the selected cell. The control gates of each cell in a row are connected to a wordline, and the wordline for a selected cell is connected to the predetermined select voltage during reading of the selected cell.
One of the problems encountered during use of sense amplifiers has been the capacitance of the drain-column line. If the drain-column-line capacitance is allowed to discharge during the time between readings, and a selected memory cell 10 has a charged (programmed "0") floating gate, then the initial surge of current through prior-art sense amplifier as the drain-column-line capacitance is charged. The initial surge of current either (a) causes a read error at the output of prior-art sense amplifier or (b) makes it necessary increase the access time to allow the drain-column line to be charged before reading selected cell. A prior-art method of preventing the drain-column line from discharging has been to charge the drain-column line through use of pre-charge circuitry. While use of pre-charge circuitry improves access time, the disadvantage of that use is that such circuitry draws a large direct current from supply-voltage source.